Distinção entre textura spinifex e arranjos de olivina metamórfica

Abstract

Elongated blades of olivine in a talc-carbonate (or sulphide) matrix are form ed by me - tamorphi c processes from serpentinites, and have been misidentifi ed as igneous spinifex texrures. The two types may be dislinguished texturally and chemically. Metamorphic olivines show straight borders and the crystals overlap in poiquiloblastic habit , whereas volcanic olivin es are often skeletal and crystals do nct overlap. Whenever crystals are not entirely serpentinlzed, those of metamorphi c origin will show negative elongation , and igneous crystals will show positive elonga tion. Fo contem is strongly control ted by the FeJMg ratio of the rock . Samples with elongated metamorphic olivines in a talc matrix contai n approx imately 40-43% MgO, as opposed 10spinifex textured igneous samples which show less than 33% MgO. Ti02 is much lower in olivíne-talc metamorphic rocks.